A. Field of the Invention
The designation “value document” is to be understood within the framework of the invention to refer to bank notes, checks, shares, tokens, identity cards, credit cards, passports and also other documents as well as labels, seals, packages or other elements for product authentication.
B. Related Art
The safeguarding of value documents against forgery by means of luminescent substances has been known for some time. The employment of transition metals and rare earth metals as luminescent ions in host lattices has already been discussed. Such ions have the advantage that, after suitable excitation, they show one or several characteristic narrow-band luminescences which facilitate a reliable detection and the delimitation over other spectra. Combinations of transition metals and/or rare earth metals have also been discussed. Such substances have the advantage that, in addition to the above-mentioned luminescences, there are observed so-called energy transfer processes which can lead to complicated spectra. In such energy transfer processes an ion can transfer its energy to another ion, and the spectra can then consist of several narrow-band lines which are characteristic of both ions.
Ions with characteristic properties which are suited for safeguarding value documents are limited in number, however. Moreover, ions of the transition metals and/or rare earth metals luminesce at one or several characteristic wavelengths which are dependent on the nature of the ion and of the host lattice and can be predicted. Energy transfer processes also lead to these characteristic luminescences of the involved ions.
With all known security elements and the luminescent substances employed for the security elements, however, a detection of the security elements or of the contained luminescent substances is done by exploiting the fact that their emissions, i.e. their characteristic luminescences, are distinguishable. In particular, the emissions have different wavelengths, so that they can be uniquely identified via the respective emission wavelength. Other criteria may be for example the decay time of the emission or its intensity. Such security elements are known for example from the documents WO 2009/136921 A1, WO 2006/024530 A1, WO 2005/035271 A2, WO 81/03508 A1, WO 99/38700 A1, WO 99/38701 A1, WO 99/39051 A1 and DE 30 48 734 A1.
The described security elements for safeguarding value documents consist of individual luminophores which differ with regard to their spectral and/or temporal properties. The security elements are applied into and/or onto value documents in different forms of use. In so doing, a combination of luminophores can be employed. The emission bands of the employed luminophores represent a spectral encoding. Several different luminophores can be combined to systems, whereby the individual systems are mutually independent. The emission of the employed luminophores is also referred to as luminescence, which may involve fluorescence and/or phosphorescence.
The above-mentioned limited number of luminophores, the limited number of different spectral lines emitted thereby, makes it necessary, for a discrimination of the different luminophores, to mutually coordinate the emissions of the employed luminophores such that the emissions of the different luminescences for a value document do not overlap. Thus there are only very limited possibilities for integrating different luminophores in an individual value document such that the value document can be reliably distinguished from other value documents.
Starting out from this prior art, the invention is based on the object of stating security elements having at least two luminescent substances which are suited in particular as an authenticity marking for value documents, whereby the security elements are to offer an even higher security against attempts at forgery. Moreover, there is to be obtained an increase in the variety of distinguishable security elements.
The invention starts out from a security element having at least two luminescent substances, whereby the security element has a first and a second luminescent substance which have a substantially identical, joint emission band, whereby at least the first or the second luminescent substance, or both luminescent substances, have at least one excitation band that leads to an emission at the joint emission band only in the case of the first or the second luminescent substance.
The invention has the advantage that it is now also possible to use for safeguarding a value document luminescent substances together that could previously not be distinguished, or not distinguished reliably, by their emissions. The different excitation bands permit a distinction of the luminescent substances while the emission band is the same. Thus, the available possibilities for safeguarding value documents are substantially increased. This allows the provision of a multiplicity of additional encodings. Moreover, the fact that the luminescent substances cannot be distinguished solely by their emissions substantially improves the safeguarding effected by the security element containing the luminescent substances.
Especially advantageously, the security element can be used for safeguarding and/or encoding value documents. For example, by incorporation and/or application to the value document or by incorporation or application to authentication features for value documents.
A verification of the presence of a security element, of a value document or of an authentication feature can be effected by exciting the luminescent substances with a first radiation which has a wavelength lying substantially at one of the excitation bands that leads to an emission at the joint emission band only in the case of the first or the second luminescent substance.
This verification by excitation at different wavelengths that correspond to the different excitation bands of the jointly employed luminescent substances allows one to also be able to mutually distinguish luminescent substances that were hitherto perceived as a single luminescent substance, because they have a joint emission band.
Further advantages of the present invention can be found in the dependent claims as well as the subsequent description of embodiments with reference to figures.